With an increase in technical development and demand for mobile devices, secondary batteries as an energy source are increasingly in demand. Accordingly, a great deal of studies and investigation into batteries to satisfy various consumers requests has been recently conducted.
For instance, in terms of morphology of a battery, a demand for an angular type secondary battery and/or a pouch type secondary battery with a small thickness being applicable to electronic products such as a cellular phone is increasing, while a secondary lithium battery having high energy density, discharge voltage, output stability, etc. such as a lithium ion battery, a lithium ion polymer battery and the like is in relatively great demand in terms of raw materials.
In addition, the secondary battery may be classified in terms of structure of an electrode assembly having the cathode/separation membrane/anode structure. For instance, a jelly-roll (winding type) electrode assembly having a structure wherein cathodes and anodes in extended sheet forms are wound by interposing a separation membrane therebetween, a stack/folding type (lamination type) electrode assembly having a structure wherein cathodes and anodes are stacked by interposing a separation membrane to fabricate a bi-cell or full cell in a laminate form and a plurality of the bi-cells or full cells are wound, and the like may be representative of the secondary batteries. Such a stack/folding type electrode assembly was described in detail in Korean Patent Laid-Open Publication Nos. 2001-0082058, 2001-0082059 and 2001-0082060 issued to the present applicant.
Among the techniques described above, the jelly-roll type electrode assembly has merits of simple and easy production and relatively high energy density per weight. However, since this assembly is generally fabricated by winding a cathode and an anode under a compact condition, which each has an extended sheet form, to constitute a structure in a cylindrical or elliptical shape so that stress generated by expansion and contraction of an electrode during charge/discharge may be accumulated in the electrode assembly and, if such stress accumulation exceeds a constant limit, the electrode assembly may become deformed. For this reason, a gap between adjacent electrodes is irregular so that performance of a battery having the electrodes may be rapidly deteriorated and internal short-circuit may occur, causing a problem of threatening safety of the battery.
Accordingly, in order to prevent deformation of a jelly-roll type electrode assembly, the present inventors proposed use of an organic/inorganic composite membrane and a method for fabrication of a jelly-roll assembly with excellent production workability because of low friction coefficient between a mandrel and a separation membrane.
In this regard, some of conventional technologies concerning organic/inorganic composite porous separation membranes have been disclosed. For instance, Korean Patent Laid-Open Publication Nos. 2007-0055979 and 2006-0050976 issued to the present applicant described an organic/inorganic composite porous membrane which includes inorganic particles on both sides of the membrane and pores formed by the inorganic particles, so as to solve thermal safety problems of a polymer separation membrane.
However, such an organic/inorganic composite porous membrane fabricated according to any conventional technique described above may cause the following problems when the membrane is adopted for a jell-roll type electrode assembly, although the membrane may be useful for a stack/folding type electrode assembly.
In general, as to manufacture of a jelly-roll type electrode assembly, a process of inserting one end of an cathode/separation membrane/anode laminate into a mandrel, winding the laminate around the mandrel in a cylindrical form, and then, removing the mandrel is involved. However, during removal of the mandrel, due to friction between the membrane, which was positioned in the innermost layer of the laminate, and the mandrel, a sheet including the electrode or the membrane may partially escape the wound electrode assembly (“tail out condition”) or other failures may occur, leading to lowered safety of the battery. More particularly, using a separation membrane with an organic/inorganic coating film may increase friction between the mandrel and the membrane, causing significant problems such as pushing off of the membrane and/or tail out condition thereof.
So as to overcome these problems, Korean Patent Laid-Open Publication No. 2007-0000231 described a novel technique characterized in that a coating layer is formed on one side of an organic/inorganic composite porous separation membrane through roll-coating, and then, the coating layer is wound to orient an electrode rather than a mandrel. However, according to practical experiments conducted by the inventors, it was determined that the above technique may cause damage to inorganic particles or a porous material due to pressure applied during the coating process through roll-coating, entailing problems such as difficulties in management and complicated production processes. In addition, an organic/inorganic composite film is applied to only one side of the membrane, which in turn exhibits lowered buffer effects on contraction and expansion of a jelly-roll structure, thus not desirably satisfying prevention of the jelly-roll structure.
Consequently, there is a strong need for development of improved techniques for enhancing safety of a battery and for inhibiting damage to a separation membrane and/or a decrease in production efficiency during manufacture of a jelly-roll structure while favorably preventing deformation of the jelly-roll structure, thereby extending battery life.